Maximum demand (often referred to as MD)
is the largest current normally carried by circuits, switches
and protective devices; it does not include the levels of
current flowing under overload or short circuit conditions,
Assessment of maximum demand is sometimes straightforward.
For example, the maximum demand of a 240 V single-phase
8 kW shower heater can be calculated by dividing the power
(8 kW) by the voltage (240 V) to give a current of 33.3
A. This calculation assumes a power factor of unity, which
is a reasonable assumption for such a purely resistive load.

There are times, however, when assessment
of maximum demand is less obvious. For example, if a ring
circuit feeds fifteen 13 A sockets, the maximum demand clearly
should not be 15 x 13 = 195 A, if only because the circuit
protection will

not be rated at more than 32
A. Some 13 A sockets may feed table lamps with 60 W lamps
fitted, whilst others may feed 3 kW washing machines; others
again may not be loaded at all. Guidance is given in {Table
6.1}.

Lighting circuits pose a special problem
when determining MD. Each lamp-holder must be assumed to
carry the current required by the connected load, subject
to a minimum loading of 100 W per lampholder (a demand of
0.42 A per lampholder at 240 V). Discharge lamps are particularly
difficult to assess, and current cannot be calculated simply
by dividing lamp power by supply voltage. The reasons for
this are:

1. - control gear losses result
in additional current,

2.
- the power factor is usually less than unity so
current is greater, and

3. - chokes and other control gear
usually distort the waveform of the current so that it contains
harmonics which are additional to the fundamental supply
current.

So long as the power factor of a discharge
lighting circuit is not less than 0.85, the current demand
for the circuit can be calculated from:

current (A) =

lamp power (W) x 1.8

supply voltage (V)

For example, the steady state
current demand of a 240 V circuit supplying ten 65 W fluorescent
lamps would be:

I =

10 x 65 x 1.8 A

= 4.88A

240

Switches for circuits feeding
discharge lamps must be rated at twice the current they
are required to carry, unless they have been specially constructed
to withstand the severe arcing resulting from the switching
of such inductive and capacitive loads.

Table
6.1 - Current demand of outlets

Type of outlet

Assumed current demand

2 A socket outlet

At least 0.5A

Other socket outlets

Rated current

Lighting point

Connected load, with
minimum of 100 W

Shaver outlet, bell transformer
or any equipment of 5 W or less

May be neglected

Household cooker

10A + 30% of remainder
+ 5A for socket in cooker unit

When assessing maximum demand,
account must he taken of the possible growth in demand during
the life of the installation. Apart from indicating that
maximum demand must be assessed, the Regulations themselves
give little help. Suggestions for the assumed current demand
of various types of outlet are shown in {Table 6.1}.